Method and apparatus for inspection of light emitting semiconductor devices using photoluminescence imaging

ABSTRACT

A method and apparatus for the inspection of light emitting semiconductor devices. The semiconductor device is illuminated with a light source, wherein at least an area of the light emitting semiconductor is illuminated with a waveband of light. The waveband of light λA+λB can generate electron-hole pairs in the light emitting semiconductor to be inspected. Through an objective lens at least a part of the light λC emitted by the light emitting semiconductor is detected. The emitted light is captured with a sensor of a camera that is sensitive to wavelengths of the emitted light, wherein the wavelength of the emitted light is above the width of the waveband. The data of the emitted light, captured with the sensor, are transmitted to a computer system for calculating inspection results of the light emitting semiconductor.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims priority of U.S. provisional patentapplication No. 61/500,987 filed Jun. 24, 2011, the application isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for inspection of lightemitting semiconductor devices during and after a production process.The light emitting semiconductor devices can be LEDs.

The present invention also relates to an apparatus for inspection oflight emitting semiconductor devices on a substrate.

BACKGROUND OF THE INVENTION

Solid state lighting (SSL) has several advantages compared toconventional lighting. Main advantages are low power consumption, longlifetime, and small form factor. An important element of SSL is the LED(Light-Emitting Diode) die/chip. Basis for LEDs is a semiconductormaterial that is undergoing a complex production process in order toobtain a LED. Several metrology and inspection steps are done during andafter that production process.

Measuring the output power of an LED is typically done using a probingsystem. In this system, electrical contacts are made to each LED die anda measurement is done of a.o. the generated light output power andoptionally the wavelength.

The international patent application WO98/11425 discloses a method andapparatus for detecting defects in a semiconductor or silicon structureat room temperature, and in an efficient time, using photoluminescence.The invention employs the use of a high intensity beam of lightpreferably having a spot size between 0.1 mm-0.5 μm and a peak oraverage power density of 10⁴-10⁹ W/cm² with a view to generating a highconcentration of charge carriers, which charge characters detect defectsin a semiconductor by interacting with same. These defects are visibleby producing a photoluminescence image of the semiconductor. Severalwavelenghts may be selected to identify defects at a selective depth aswell as confocal optics are used. This method performs probing of a verysmall volume of the material with one or more laser beams having verysmall spot size.

Another method is described in U.S. Pat. No. 7,504,642 B2 where one ormore images are created using filtering and image computation toselectively create a defect image of one selected layer of a wafer,while trying to eliminate unwanted contributions of other layers of thesame wafer. The method uses photoluminescence to identify defects in oneor more specified material layers of a sample. One or more filteringelements are used to filter out predetermined wavelengths of returnlight emitted from a sample. The predetermined wavelengths are selectedsuch that only return light emitted from one or more specified materiallayers of the sample is detected. Additionally or alternatively, thewavelength of incident light directed into the sample may be selected topenetrate the sample to a given depth, or to excite only one or moreselected material layers in the sample. Accordingly, defect datacharacteristic of primarily only the one or more specified materiallayers is generated.

The international patent application WO 2007/128060 A1 describes aphotoluminescence based method for testing of indirect bandgap (e.g. Si)semiconductor materials, based on comparison of several regions in twoor more images. The method is suitable for identifying or determiningspatially resolved properties in indirect bandgap semiconductor devicessuch as solar cells. In one embodiment, spatially resolved properties ofan indirect bandgap semiconductor device are determined by externallyexciting the indirect bandgap semiconductor device to cause the indirectbandgap semiconductor device to emit luminescence, capturing images ofluminescence emitted from the indirect bandgap semiconductor device inresponse to the external excitation, and determining spatially resolvedproperties of the indirect bandgap semiconductor device based on acomparison of relative intensities of regions in one or more of theluminescence images.

Quality control of LED is becoming more and more crucial since LEDs areused for illumination. It is important that, e.g., the LEDs used for theback illumination of a TV set are of similar intensities. Therefore aquality control of the light output power of LEDs needs to be done. Suchquality control was until now done by electrically contacting the LED(probing) and then measuring the emitted light output power. This hasseveral disadvantages: LEDs may get damaged during probing, probing isslow and requires an additional tool.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for quickly andreliably measure the light power emitted by an LED during a productionprocess. Furthermore, the method should be easy to use and should notinfluence or destroy the LED to be measured.

The object is achieved by a method for inspection of light emittingsemiconductor devices, comprising the following steps:

-   -   illuminating with a light source at least an area of the light        emitting semiconductor with a waveband of light, wherein the        waveband of light λ_(A)+λ_(B) can generate electron-hole pairs        in the light emitting semiconductor to be inspected;    -   detecting through an objective lens at least a part of the light        λ_(C) emitted by the light emitting semiconductor, wherein the        emitted light is captured with a sensor of a camera that is        sensitive to wavelengths of the emitted light and the wavelength        of the emitted light is above the a width of the waveband; and    -   transferring data of the emitted light, captured with the        sensor, to a computer system for calculating inspection results        of light emitting semiconductor.

It is a further object of the invention to provide an apparatus thelight power emitted by an LED during a production process. Furthermore,the apparatus should be easy to use and should not influence or destroythe LED to be measured.

The object is achieved by an apparatus for inspection of light emittingsemiconductor devices on a substrate, comprising:

-   -   a light source;    -   an objective lens defining a detection beam path;    -   a camera with a sensor positioned in the detection beam path,        for receiving light from the light emitting semiconductor        devices via the objective lens; wherein the sensor registers        grey scale values of the light emitting semiconductor devices;    -   a computer system for calculating a wafermap from data registers        by the sensor; and    -   a display in order to visually display the wafermap.

For defect inspection the photoluminescence effect is used as a kind ofback light. This illumination effect enables the finding of defects thatare buried or at least are not visible in normal inspection. With theinventive setup it is as well possible to find cuts (finger cuts) orinterrupts in the metallization layer. Additionally, the inventionallows the detection of non-homogeneities of the LEDs. The light emittedfrom the LED is subjected to a spatial grey value analysis. Some LEDsonly emit light in some parts while no light is emitted in other parts(for example: a dark edge at the corner of the LED does not light up).

According to an embodiment of the invention the light emittingsemiconductor devices are illuminated with a light source configured asa ring light. The ring light has a plurality of LEDs. A second filtercan be positioned in the detection beam path. The second filter in thedetection beam path prohibits the reflections of the incident light toreach the sensor and at least a wavelength of λC passes the secondfilter.

A further embodiment of the invention is that a first filter ispositioned in an illumination beam path of the light source and isdesigned to pass a waveband λA+λB. The second filter is in the detectionbeam path and prohibits the reflections of the incident light to reachthe sensor and at least a wavelength of λC passes the second filter. Inthis case the objective lens defines the illumination beam path and thedetection beam path. The light source is a coaxial light source.

One or several LED die/chips are illuminated with a light source withwavelengths that can generate electron hole pairs in the LED. The lightemitted by the LED (caused by the electron hole pair and followingrecombination process) is captured with a sensor/camera that issensitive to the wavelengths of the emitted light. The sensor response(gray value) is a measure for the power of the light output of the LEDand can for example be used to classify the LEDs according to theirlight output power.

The waveband of light λA±λB for the illumination of light emittingsemiconductor devices or the LED die/chips is generated by inserting afirst filter prior to the objective lens in an illumination beam path. Asecond filter is positioned in a detection beam path after the objectivelens, so that only the light emitted by the light emitting semiconductorreaches the sensor of the camera. The image acquisition setup,especially the objective lens comprises microscope optics. Various typesof illumination can be used in the apparatus for the illumination oflight emitting semiconductor devices or the LED die/chips. The lightsource could be a coaxial light source or a ring light. The illuminationlight is provided with a plurality of LEDs.

The inventive method is applied to LED die/chips, which are structureson a substrate or wafer. The inspection result is then a measure for alight output power of a LED or the LED die/chips detected by the sensorof the camera. The output of the sensor is at least one gray value of amatrix of pixels. A range of the gray value establishes function of thelight output power per LED in the LED die/chips or in the light emittingsemiconductor devices.

The inventive apparatus has a stage, which moves the substrate with theLED die/chips in a X/Y direction. The movement is controlled by thecomputer system. With the relative movement between the camera and thesubstrate, the sensor of the camera captures an image of the entiresurface of the substrate. The data from the sensor is sent to thecomputer system which calculates a wafermap of the surface with the LEDdie/chips. The wafermap is shown on a display of the computer system,wherein to each class of gray value a separate color code is assigned.

The function of the light output power per LED is implemented as a lookup table. A further embodiment is that the function is implemented as apolynomial. A calibration of look up table or the polynomial is done bymeasuring the light output power of an LED sample by connecting it to anelectrical prober.

The inspection result generated by the sensor is at least one gray valueper LED. The inspection results of the LEDs are sorted in at least twobins according to their registered gray value. In a further form, theinspection result generated by the sensor is at least two gray valuesper LED die/chip. The variations or differences in gray value of one LEDdie/chip are used as a quality measure of the LED die/chip.

The inspection result is at least one gray value per LED die/chip andthe method comprises the steps of:

-   -   taking at least two inspection images under same conditions of        each LED die/chip;    -   taking the images under varying illumination intensity and/or        exposure settings which are configurable;    -   generating a histogram of the gray values is generated for each        LED die; and    -   analyzing the histogram distribution in order to establish a        pass or a fail criteria.

As mentioned before, the light emitting semiconductor devices are LEDdie/chips and the emitted light of the LED is caused by a recombinationprocess of electron hole pairs that are generated by the illumination inan active layer of the LED. The emitted wavelength or waveband has asimilar wavelength or waveband as if a forward voltage would be appliedto the LED.

Due to fluctuations in the production process, the LED chips are sortedaccording to several criteria including center wavelength of the emittedlight, power of the emitted light, etc.

This invention would allow for fast and contactless inspection on aninspection tool which is widely used by LED manufactures for otherinspection tasks.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature and mode of operation of the present invention will now bemore fully described in the following detailed description of theinvention taken with the accompanying drawing figures, in which

FIG. 1 is a table showing the bandgap and corresponding wavelength of aIII-N semiconductor material system;

FIG. 2 is a typical layer stack of an LED;

FIG. 3 is an embodiment of the apparatus according to the invention forilluminating one or several LED die/chips in order to detect the emittedlight from the LED die/chips;

FIG. 4 is a further embodiment of the apparatus according to theinvention for illuminating one or several LED die/chips in order todetect the emitted light from the LED die/chips;

FIG. 5 is a simplified view of a wafermap generated with the inventiveapparatus.

FIG. 6 a is an image of the surface of a wafer with LED die/chips whichare illuminated with normal illumination;

FIG. 6 b is an image of the InGaN—layer below the surface of the LEDdie/chips on a wafer which is illuminated with photo luminescenceillumination; and

FIG. 7 is a schematic image of the wafermap showing an image of the LEDdie/chips in the photoluminescence setup.

DETAILED DESCRIPTION OF THE INVENTION

Same reference numerals refer to same elements throughout the variousfigures. Furthermore, only reference numerals necessary for thedescription of the respective figure are shown in the figures. The shownembodiments represent only examples of how the invention can be carriedout. This should not be regarded as limiting the invention. Thedescription below refers to LED die/chips which should not be regardedas a limitation of the invention. It is evident for any person skilledin the art that the present invention is applicable to light emittingsemiconductor material in general.

FIG. 1 is a table 100 showing the bandgap and corresponding wavelengthof a III-N semiconductor material system. All semiconductor materialsexhibit the so-called photoluminescence effect. This effect is seen whenthe material is illuminated with light of a certain wavelength, and thephotons in the light beam will bring electrons from a low energy stateto a high energy state (will generate electron-hole pairs). This iscalled photo-excitation. The incoming light beam should have an energylevel above the difference between high-energy state and low energystate. This is typically the bandgap energy of the semiconductormaterial. The generated pairs will recombine and the recombinationprocess will generate photons (radiative recombination) or phonons(non-radiative recombination). In most LED materials (which are directsemiconductors) such as the GaN system, the radiative recombinationprocess is the dominant one.

FIG. 2 is a typical representation of a layer stack 101 of an LED. Thelayer stack 101 has the substrate 3 on which a layer 102 of n-type GaNis formed. The layer 102 of n-type GaN carries an intermediate layer 103of a InGaN MQW material. A top layer 104 is formed by a p-type GaNmaterial. In order to probe only the intermediate layer 103 of InGaNMQW, the excitation light 110 should not be absorbed by the layer 102 ofn-type GaN and the top layer 104 of p-type GaN surrounding it. Theexcitation light 110 should have an energy level below the GaN energyband level, meaning wavelength above 359 nm. For the light to beabsorbed by the intermediate layer 103 of InGaN MQW material, theexcitation light 110 should have an energy level above 2.75 eV i.e.below 450 nm. The generated light 120 by the intermediate layer 103 ofInGaN MQW material will have a wavelength around 450 nm. In theapparatus 1, described in FIG. 3, a white light source 7 is used.Consequently, the energy levels below 450 nm in the light path have tobe filtered out. This means a first low pass filter 15 (pass onlywavelength <450 nm). In order to make clear images and not to bedisturbed by the reflections of the incoming light, an additional secondfilter is needed in detection beam path 21 with high-passcharacteristics i.e. pass only wavelength of 450 nm and higher.

FIG. 3 is a schematic representation of an embodiment of the apparatus 1for illuminating one or several LED die/chips 5 on a substrate 3 inorder to detect the wavelengths of the emitted light from the LEDdie/chips 5. One or several LED die/chips 5 are illuminated with a lightsource 7 with wavelengths that can generate an electron hole pairs inthe LED. The light emitted by the LED (caused by the electron hole pairand following recombination process) is captured with a camera 9 that issensitive to the wavelengths of the emitted light. The camera 9 has asensor 10 and the response (gray value) of the sensor 10 is a measurefor the power of the light output of the LED and can for example be usedto classify the LEDs according to their light output power.

The light source 7 is a white-light broadband spectrum light source, isused for illuminating the substrate 3 with the LED die/chips 5. Thelight from the light source 7 supplied to a microscope 6 via a lightguide 8. The microscope 6 defines an illumination beam path 11. A beamsplitter 12 directs the illumination beam path 11 via an objective lens14 onto the LED die/chips 5 on the substrate 3. By providing means (notshown) for inserting a respective first filter 15 in the illuminationbeam path 11, a certain part of the broadband spectrum that is generatedby a light source 7, is selected. The light is transmitted through theobjective lens 14 (incident light beam) excites the semiconductormaterial in the LED die/chips 5 on the substrate 3. This could be forexample a direct band gap material such as a III-V semiconductormaterial as used for LED fabrication. The semiconductor material willemit light at a known wavelength and this light is collected in the sameobjective lens 14. The objective lens 14 defines as well a detectionbeam path 21.

In the detection beam path 21 a second filter 16 is positionable inorder to make sure that only the light emitted by the LED die/chips 5 onthe substrate 3 reach the camera 9 and the sensor 10. The second filter16 prohibits the reflections of the incident light to reach the camera 9or the sensor 10. The image data collected by the sensor 10 of camera 9are fed to a computer system 17 which uses an image processing softwareto derive an average intensity for each LED 4 on the substrate 3. Thecomputer system 17 computes a wafermap (see FIG. 4). A display 18 isassigned to the computer system 17 in order to visually display wafermap30 to plot the results of all LEDs 4 and their coordinate position onthe substrate 3, which is in many cases a wafer.

The light source 7 is coaxial light source. It is advantageous if thelight source 7 is ring light. The illumination light is provided by aplurality of LEDs. The light source 7 is configured as a pulsed lightsource or a continuous light source. The waveband constraint (λA+λB)<λCis implemented using optical high pass and/or low pass and/or band passfilters. The sensor 10 is a line sensor. The camera 9 is configured as aTDI (time delay integration) line scan camera. The sensor 10 could be aswell a 2-dimensional sensor so that an area scan camera results.

The emitted light of the LED die/chips 5 or LED 4 is caused by therecombination process of electron hole pairs that are generated by theillumination. The emitted light of the LED die/chips 5 or LED 4 iscaused by the recombination process of electron hole pairs that aregenerated by the illumination which has a similar wavelength as if aforward voltage would be applied to the LED die/chips 5 or LED 4. Therecombination process takes place in the active layer of the LEDdie/chips 5 or LED 4. In case of a blue LED an example implementationwould be λA≈380 nm, λB≈20 nm and λC≈440 nm.

A calibration is done to correlate the measured average intensity of theLED material to an output power (density) number. The inventiveapparatus 1 uses a white-light source with area illumination. Prior artdevices instead use a commonly laser beam source with small spot size,and a camera as a detector. The computer system 17 controls as well aX/Y-stage 19. The X/Y-stage 19 moves the substrate 3 in a controlledmanner so that the entire surface of the substrate is imaged by theobjective lens 14 onto the sensor 10 of camera 9. The position of theX/Y-stage 19 is recorded in order to correlate the visually captureddata with the position data on the substrate 3 and to generate thewafermap 30.

FIG. 4 is a further embodiment of the apparatus 1 for illuminating oneor several LED die/chips 5 on a substrate 3 in order to detect thewavelengths of the emitted light from the LED die/chips 5. According tothe embodiment shown here the LED die/chips 5 are illuminated with alight source 7 which is configured as a ring light source. The ringlight source comprises several LEDs which emit wavelengths that cangenerate an electron hole pairs in the LED die/chips 5 on a substrate 3.The light emitted by the LED (caused by the electron hole pair andfollowing recombination process) is captured with a camera 9 that issensitive to the wavelengths of the emitted light. The camera 9 has asensor 10 and the response (gray value) of the sensor 10 is a measurefor the power of the light output of the LED and can for example be usedto classify the LEDs according to their light output power.

The ring light source defines an illumination 11 by which a certain areaon the LED die/chips 5 on a substrate 3 illuminated. The embodimentshown in FIG. 4 does not need first filter 15 for the illumination 11,of the surface of the LED die/chips 5. The LEDs of the ring light sourceare driven in such a way that the required light is emitted in order togenerate the electron hole pair in the semiconductor material. Thesemiconductor material will emit light at a known wavelength and thislight is collected by the objective lens 14. The objective lens 14defines as well a detection beam path 21.

FIG. 5 is a simplified view of a wafermap 30 generated with theinventive apparatus 1. The X/Y-stage 19 is moved so that an entire imageof the surface 3 a of the substrate 3 (wafer) is obtained. The computersystem 17 stitches the individual images, taken with the objective lens14, together in order to get a representation of the entire surface 3 aof the substrate 3 (wafer). In case of the layer stack 101 of an LED asshown in FIG. 2 the intermediate layer 103 of InGaN MQW is visible withthe inventive apparatus 1. The intermediate layer 103 of InGaN MQW isnow visible below the top layer 104 of p-type GaN. The wafermap 30 iscomputed to plot the results of all LEDs 4 on their coordinate positionon the substrate 3 (wafer). A calibration is done to correlate themeasured average intensity of the LED material to an output power(density) number. The representation can be done using different greyscales. An image of the surface 3 a of the substrate 3 (wafer) is takenduring inspection with an inserted first filter 15 and second filter 16.A spot size (not shown) of the illumination light can be larger than thesize of the LED die/chips 5, thus it is possible to illuminate the wholeLED 4 and subsequently the related, often subsequent, measurement is acorrect representation of the characteristics of the whole LED die/chips5.

FIG. 6 a is an image of the surface 3 a of a substrate 3 (wafer) withthe LED die/chips 5 which is illuminated with normal illumination (whitelight). The image of the surface 3 a of a substrate 3 (wafer) with theLED die/chips 5 is taken using standard illumination. With thisillumination all LED die/chips 5 appear to be identical. FIG. 6 b is animage of the surface 3 a of a substrate 3 (wafer) with the LED die/chips5 wherein the surface 3 a is illuminated with the first filter 15 inillumination beam path 11 and the image is captured with the secondfilter 16 in the detection beam path 21. Due to the photo luminescencethe surface 3 a of a substrate 3 (wafer) shines in blue light, which isgenerated by the LED die/chips 5. It is clear from the comparison ofFIG. 6 a and FIG. 6 b that with the photo luminescence setup, inspectionfeatures become visible, that are invisible for the “normal” or standardillumination setup (white light). The intermediate layer 103 of InGaNMQW is clearly visible below the surface or the top layer 104 of p-typeGaN. Circles 51 with dashed lines indicate LED die/chips 5 withidentical appearance under normal illumination (white light), butwithout response under the photo luminescence setup. All LED die/chips 5are having the same grey scale value (GV) when using the standardillumination setup (white light), the LED die/chips 5 can have asignificantly different GV response when using the photo luminescencesetup.

FIG. 7 is a screenshot of the wafermap 30 showing an image of the LEDdie/chips 5 in the photoluminescence setup on the display 18. Usingsoftware which is implemented in the computer system 17(see FIG. 3 or 4)an inspection of the properties of LEDs or LED die/chips 5 images ispossible. This means that it is possible to locate the individual LEDdie/chips 5 on the images, measure certain properties based on imageprocessing, and then correlate the measurement results to eachindividual LED die/chip 5. A recipe is set up with rule-based binning(“RBB”). According to the recipe a classification of the LED die/chips 5according to the average GV of the entire LED die/chip 5 on thesubstrate 3 is carried out. Each class has a separate color code. In aseparate section 31 of the display 18 the various GVs are shown in ahistogram 32, which results from the rule-based binning The inspectionof the substrate 3 (wafer) with the LED die/chips 5 shows a signature,that it is possible with the photo luminescence to measure somethinggenuinely different from what can be seen with normal inspection setup.It can also be seen that the response of individual LED die/chips 5,which can be neighboring, can be independent of the wafer-levelsignature. It is a clear indication that measurement on the die—level isa big additional source of information in the process improvement of LEDmanufacturing. With the wafermap 30 it can be shown that with ameasurement on a partly or fully processed substrate 3 (wafer) withLEDs, using a photoluminescence setup, a quantitative indication of theexpected output power for each individual LED is obtained.

The inventive method is suitable for inspecting at least one LEDdie/chip 5 or more general a light emitting semiconductor material,which is structured on a substrate 3 or wafer. At least the area of oneLED die/chip 5 is illuminated with a waveband (λA±λB) that can generateelectron-hole pairs in the LED die/chip 5 to be inspected. The wavebandis obtained with the first filter 15 in the illumination beam path 11.At least a part of the light emitted by the LED die/chip 5 is capturedwith the sensor 10 of the camera 9. The second filter 16 is positionablein order to make sure that only the light emitted by the LED die/chips 5on the substrate 3 reach the camera 9 and the sensor 10, so that sensor10 is sensitive to wavelengths (λC+λD) of the emitted light. Thewavelength λC is larger than the wavelengths (λA+λB). The inspectionresult is the output of the sensor 10 which is fed to the computersystem 17.

The inspection result is a measure for the light output power of an LEDor a LED die/chip 5. The output of the sensor 10 is at least one grayvalue of at least one pixel. Usually, the gray value is represented by amatrix of pixels. The range of the grey value, e.g. for an 8 bitcomputer system 17 is between 0-255 per LED die/chip 5. The output poweris a function of the measured grey values. The function can beimplemented as a look up table or as a polynomial. The calibration ofthe look up table or the polynomial is done by measuring light outputpower of a LED sample when connected to an electrical prober.

The inspection result is at least one gray value per LED die/chip 5. TheLEDs are sorted in at least two bins according to their gray value (atleast one threshold value) here the inspection result is at least twogray values per LED die/chip 5, the variations/differences in gray valueof one LED die/chip 5 are used as a quality measure of the LED die/chip5.

The inspection result is at least one gray value per LED die/chip 5, ofeach LED die/chip 5 multiple inspection images (at least two) are takenin order to detect stability and deviations on the emitted light. Allimages can be taken under same conditions or mages can be are takenunder varying illumination intensity and/or exposure settings. Thatmeans the first inspection image is taken under condition A, the secondunder condition B, where conditions A, B, and so on are configurable.Calibration of parameters may be done using the result from electricalprober. A histogram of the gray values is generated for each LEDdie/chip 5 and a classification into pass/fail is done by analyzing thehistogram distribution. Examples: If the histogram distribution isbi-modal then fail. If the histogram distribution uni-modal and has alow gray value then fail. If the histogram distribution is uni-modal andhas a large gray value then pass. One of the methods above can be usedas pre/post check for the electrical prober.

The invention has been described with reference to specific embodiments.It is obvious to a person skilled in the art, however, that alterationsand modifications can be made without leaving the scope of thesubsequent claims.

1. A method for inspection of light emitting semiconductor devices,comprising the following steps: illuminating with a light source atleast an area of the light emitting semiconductor with a waveband oflight, wherein the waveband of light λ_(A)+λ_(B) can generateelectron-hole pairs in the light emitting semiconductor to be inspected;detecting through an objective lens at least a part of the light λ_(C)emitted by the light emitting semiconductor, wherein the emitted lightis captured with a sensor of a camera that is sensitive to wavelengthsof the emitted light and the wavelength of the emitted light is abovethe a width of the waveband; and transferring data of the emitted light,captured with the sensor, to a computer system for calculatinginspection results of light emitting semiconductor.
 2. The method ofclaim 1, wherein the light source illuminates the area of the lightemitting semiconductor directly with the waveband of light λ_(A)+λ_(B)for illumination which is generated by a plurality of LEDs of the lightsource.
 3. The method of claim 2, wherein a second filter is positionedin a detection beam path after the objective lens, so that only thelight emitted by the light emitting semiconductor reaches the sensor ofthe camera.
 4. The method of claim 1, wherein the light sourceilluminates the area of the light emitting semiconductor through anobjective lens, wherein the waveband of light λ_(A)+λ_(B) forillumination is generated by inserting a first filter prior to theobjective lens in an illumination beam path.
 5. The method of claim 4,wherein a second filter is positioned in a detection beam path after theobjective lens, so that only the light emitted by the light emittingsemiconductor reaches the sensor of the camera.
 6. The method of claim1, wherein the light emitting semiconductor are a plurality of LEDdie/chips structured on a substrate and the inspection result is ameasure for a light output power of a LED in the LED die/chips detectedby the sensor.
 7. The method of claim 6, wherein the output of thesensor is at least one gray value of a matrix of pixels and a range ofthe gray value is a function of the light output power per LED in theLED die/chips.
 8. The method of claim 6, wherein a stage is moving thesubstrate with the LED die/chips and the sensor of the camera capturesan image of the entire surface of the substrate and a wafermap of thesurface with the LED die/chips the is shown on a display of the computersystem, wherein to each class of gray value a separate color code isassigned.
 9. The method to claim 7, wherein the function is implementedas a look up table.
 10. The method to claim 7, wherein the function isimplemented as a polynomial.
 11. The method of claim 9 or 10, whereincalibration of look up table or the polynomial is done by measuringlight output power of an LED sample by connecting it to an electricalprober.
 12. The method of claim 6, wherein the inspection result is atleast one gray value per LED and the inspection results of the LEDs aresorted in at least two bins according to their gray value.
 13. Themethod of claim 6, wherein the inspection result is at least two grayvalues per LED die/chip, the variations or differences in gray value ofone LED die/chip are used as a quality measure of the LED die/chip. 14.The method of claim 6, wherein the inspection result is at least onegray value per LED die/chip and the method comprises the steps of:taking at least two inspection images under same conditions of each LEDdie/chip; and/or taking the images under varying illumination intensityand/or exposure settings which are configurable; generating a histogramof the gray values is generated for each LED die; and analyzing thehistogram distribution in order to establish a pass or a fail criteria.15. An apparatus for inspection of light emitting semiconductor deviceson a substrate, comprises: a light source; an objective lens defining adetection beam path,; a camera with a sensor positioned in the detectionbeam path, for receiving light from the light emitting semiconductordevices via the objective lens ; wherein the sensor registers grey scalevalues of the light emitting semiconductor devices; a computer systemfor calculating a wafermap form data registers by the sensor; and adisplay in order to visually display the wafermap.
 16. The apparatus ofclaim 15, wherein a second filter is positioned in the detection beampath.
 17. The apparatus of claim 16, wherein the second filter in thedetection beam path prohibits the reflections of the incident light toreach the sensor and at least a wavelength of λ_(C) passes the secondfilter.
 18. The apparatus of claim 17, wherein the light source is aring light.
 19. The apparatus of claim 18, wherein the ring light has aplurality of LEDs.
 20. The apparatus of claim 15, wherein a first filterin an illumination beam path of the light source is designed to pass awaveband λ_(A)+λ_(B) and the second filter in the detection beam pathprohibits the reflections of the incident light to reach the sensor andat least a wavelength of λ_(C) passes the second filter.
 21. Apparatusof claim 20, wherein the light source is a coaxial light source. 22.Apparatus of claim 15, wherein the light source is a pulsed light sourceor a continuous light source.
 23. Apparatus of claim 20, wherein thewaveband constraint (λ_(A)+λ_(B))<λ_(C) is implemented using opticalfirst filter and second filter.
 24. Apparatus of claim 15, wherein astage, movable in the X/Y-direction is provided and the computer systemprovides a controlled movement of the stage so that an entire surface ofthe light emitting semiconductor devices on the substrate is imaged viathe objective lens in the sensor of the camera.
 25. Apparatus of claim15, wherein the light emitting semiconductor devices are LED die/chipsand the emitted light of the LED is caused by a recombination process ofelectron hole pairs that are generated by the illumination in an activelayer of the LED and has a similar wavelength as if a forward voltagewould be applied to the LED.
 26. An apparatus for inspection of lightemitting semiconductor devices on a substrate comprises: a light source,configured as a ring light source; an objective lens defining adetection beam path; a camera with a sensor positioned in the detectionbeam path, for receiving light from the light emitting semiconductordevices via the objective lens and a second filter; wherein the sensorregisters grey scale values of the light emitting semiconductor devices;a computer system for calculating a wafermap from data registers by thesensor; and a display in order to visually display the wafermap.
 27. Anapparatus for inspection of light emitting semiconductor devices on asubstrate, comprises: a light source which is a coaxial light source; anobjective lens defining an illumination beam path, wherein a firstfilter is positionable prior to the objective lens in the illuminationbeam path; a camera with a sensor positioned in a detection beam path,for receiving light from the light emitting semiconductor devices viathe objective lens and a second filter; wherein the sensor registersgrey scale values of the light emitting semiconductor devices; acomputer system for calculating a wafermap form data registers by thesensor; and a display in order to visually display the wafermap.